This is the 7638th most frequent Russian word.
железная
"Iron" or "made of iron."
In this sentence, 'железная' metaphorically means 'iron' to describe someone's strong and determined will.
У неё железная воля.
She has an iron will.
Here, 'железная' literally describes something made of iron material.
Этот мост сделан из железной конструкции.
This bridge is made of an iron structure.
The word 'железная' is used figuratively to imply something strict or unyielding.